In a typical fuel grade ethanol production process such as may be used in a corn ethanol plant, beer having a concentration of ethanol that is usually no more than approximately 15% by volume is directed to a distillation process where the ethanol in the beer is extracted in a fractional distillation system. Distillation columns typically have a multitude of horizontal trays for bringing rising ethanol vapor and descending liquid into contact. In a distillation column, low pressure steam percolates up through the beer as the beer cascades from higher trays to lower trays. As the rising steam heats the beer, the ethanol in the beer evaporates and rises to the top of the column where it exits as an overhead vapor. The remaining water and other grain material in the beer descends to the bottom of the column to exit as beer bottoms.
To produce fuel grade ethanol, more than one interconnected distillation column is normally used to progressively purify the ethanol product. In a typical ethanol distillation process, a beer column receives beer and produces an intermediate ethanol vapor. A rectifier column receives the intermediate ethanol vapor from the beer column and produces 190 proof (95% pure) ethanol vapor. A third, side stripper column receives rectifier bottoms from the rectifier column and produces an intermediate ethanol overhead vapor that is further purified by the rectifier column. Because ethanol and water form an azeotropic mixture, an extractive distillation process can only practically produce an ethanol water distillate that is approximately 95% ethanol and 5% water.
Reflux is the portion of the overhead vapor product from a distillation column that is returned as liquid to the upper part of the column in order to improve the separation of the lower boiling temperature material from the higher boiling temperature material. The more reflux provided to an ethanol rectifier column, the higher proof ethanol vapor will be produced by the rectifier, limited only by the azeotrope. However, the recycled reflux flow reduces the product output (“capacity”) of the rectifier column.
A dehydrator may be used downstream of an ethanol/water rectifier column to remove additional water from the overhead vapor in order to produce a higher purity product. The dehydrator may receive 95% ethanol vapor from the rectifier column and may remove nearly all of the remaining water to produce ethanol having a water content of less than 0.25%. Typically, a dehydrator contains beads of material which attract water to a greater degree than ethanol, or it contains a water permeable membrane that preferentially passes water across the membrane and simultaneously limits the passage of ethanol across the membrane.
Reflux is routinely used in the fractional distillation of both azeotropic as well as non-azeotropic mixtures. Reflux is used to reduce the number of theoretical trays necessary to obtain a target purity of distillate. In practice, an economic evaluation is conducted when designing a distillation system which takes into account both capital costs, such as the design size of the columns, and operating costs, such as the corresponding amount of reflux required for a given design.
For minimum-boiling azeotropic mixtures, such as ethanol and water, the operating line for the rectifying section of distillation has a “sway-back” at high concentrations on its Vapor Pressure Equilibrium (VLE) curve. To get a minimum-boiling azeotropic mixture such as ethanol and water close to its azeotrope, the slope of the operating line must be increased to almost 45 degrees. This requires increasing the amount of reflux liquid until it almost equals the amount of vapor flowing up through the column, thereby increasing the reflux ratio sharply. This procedure significantly limits the amount of product produced in a column of a given diameter, since most of the condensed vapors have to be recycled back to the rectifier column as reflux liquid. Consequently, it takes about twice as much energy to produce a gallon of 95 weight percent ethanol as it does to produce a gallon of 85 weight percent ethanol.
In a fractional distillation process for a minimum-boiling azeotropic mixture such as ethanol and water, the top tray of the rectifier column typically has a concentration that is close to, but lower than, the mixture's natural azeotrope. The overhead vapor from the top tray of a rectifier has a composition more near the natural azeotrope than the liquid on that top tray due to the physical properties of the azeotropic components. Subsequently, the overhead vapor is condensed and subcooled to produce liquid distillate. Because of the physical properties of an azeotropic mixture, the subcooled distillate typically has a concentration inferior to that of the overhead vapor, but superior to the concentration on the top tray of the rectifier. A portion, typically more than fifty percent (50%), of the subcooled distillate is recycled back to the top tray of the rectifier as reflux and the remainder is withdrawn as distillate product.
To date, there has been limited success in reducing the quantity of reflux necessary in the fractional distillation of a minimum-boiling azeotropic mixture. One approach that has been used is illustrated in FIG. 1. In a fuel ethanol production plant 10, a portion of an overhead vapor 12 produced in a rectifier column 14 is recycled back to the rectifier column 14 as reflux liquid 16. In order to reduce the quantity of reflux liquid needed, a high proof vapor 18, such as may be sourced downstream of a dehydration process (not shown) of the same plant 10, is recycled back to the rectifier column 14 in combination with overhead vapor 12. The overhead vapor 12 has a composition below its natural azeotrope. The high proof vapor 18 and overhead vapor 12 mix together to form a vapor mix 30 which is condensed in an indirect contact condenser 20, producing a subcooled distillate mixture 22 which in turn is directed to a distillate collection vessel 24 and circulated by pump 26. This subcooled distillate mixture 22 is of superior composition than what would otherwise be produced without the addition of the high proof vapor 18, thereby reducing the volume of reflux liquid 16 that is needed to achieve a target purity in the overhead vapor 12. Part of the superior subcooled distillate mixture 22 is withdrawn as distillate product 28 and directed to the dehydration process.
Most other work to date for decreasing the reflux requirement or improving the fractional distillation of azeotropic mixtures involves the use of entrainers which alter the thermodynamic relationship between azeotropic constituents. Entrainers have been used successfully across a wide variety of industries, but they add cost, may be difficult or dangerous to handle, and ultimately must be removed from the final product stream. Thus, other solutions for reducing the reflux requirement are desired.